Durable thermal barrier coating

ABSTRACT

A thermal barrier coating for superalloy turbine engine vanes and blades that are exposed to high temperature gas is disclosed. The coating includes an Aluminide or MCrAlY layer, an alumina layer, and a ceramic top layer. The ceramic layer has a columnar grain microstructure. A bond inhibitor is disposed in the gaps between the columnar grains. This inhibitor is either unstabilized zirconia, unstabilized hafnia, or a mixture thereof.

TECHNICAL FIELD

This invention relates generally to thermal barrier coatings forsuperalloy substrates and in particular to a multilayer, ceramic thermalbarrier coating resistant to sintering damage for superalloy blades andvanes in gas turbine engines.

BACKGROUND OF THE INVENTION

As gas turbine engine technology advances and engines are required to bemore efficient, gas temperatures within the engine continue to rise.However, the ability to operate at these increasing temperatures islimited by the ability of the superalloy turbine blades and vanes tomaintain their mechanical strength when exposed to the heat, oxidation,and corrosive effects of the impinging gas. One approach to this problemhas been to apply a protective thermal barrier coating which insulatesthe blades and vanes and inhibits oxidation and hot gas corrosion.

Typically, the thermal barrier coating will have an outer ceramic layerthat has a columnar grained microstructure. Gaps between the individualcolumns allow the columnar grains to expand and contract withoutdeveloping stresses that could cause spalling. Strangman, U.S. Pat. Nos.4,321,311, 4,401,697, and 4,405,659 disclose a thermal barrier coatingfor a superalloy substrate that contains a MCrAlY layer, an aluminalayer, and an outer columnar grained ceramic layer. Duderstadt et al.,U.S. Pat. No. 5,238,752 and Strangman copending U.S. patent applicationSer. No. 06/603,811, now U.S. Pat. No. 5,514,482, disclose a thermalbarrier coating for a superalloy substrate that contains an aluminidelayer, an alumina layer, and an outer columnar grained ceramic layer.

A problem with columnar grained ceramic layers is that when exposed totemperatures over 1100° C. (2012° F.) for substantial periods of time,sintering of the columnar grains occurs. The gaps close as adjacentcolumnar grains bond together. Once the gaps become closed, the ceramiclayer can no longer accommodate the thermal expansion and may spall orcrack.

Accordingly there is a need for a thermal barrier coating having acolumnar grained ceramic layer that is resistant to the sintering of thegrains.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a superalloy articlehaving a thermal barrier coating which includes a ceramic layer that isresistant to sintering when exposed to high temperature gas.

Another object of the present invention is to provide a method ofapplying a sintering resistant thermal barrier coating to a superalloysubstrate.

The present invention achieves these objects by providing a thermalbarrier coating for a superalloy substrate that includes an Aluminide orMCrAlY layer, an alumina layer, and a ceramic top layer. The ceramiclayer has a columnar grain microstructure. A bond inhibitor is disposedin the gaps between the columnar grains. This inhibitor is eitherunstabilized zirconia, unstabilized hafnia, or a mixture thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross sectional schematic of a coated article ascontemplated by the present invention.

FIG. 2 is an enlargement of a portion of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to the drawing, a base metal or substrate 10 is a nickel,cobalt or iron based high temperature alloy from which turbine airfoilsare commonly made. Preferably, the substrate 10 is a superalloy havinghafnium and/or zirconium such as MAR-M247, IN-100 and MAR-M 509, thecompositions of which are shown in Table 1.

                                      TABLE 1                                     __________________________________________________________________________    Alloy Mo W Ta                                                                              Al                                                                              Ti Cr Co Hf                                                                              V Zr C B  Ni                                        __________________________________________________________________________    Mar-M247                                                                            .65                                                                              10                                                                              3.3                                                                             5.5                                                                             1.05                                                                             8.4                                                                              10 1.4                                                                             --                                                                              .055                                                                             .15                                                                             .15                                                                              bal.                                      IN-100                                                                              3.0                                                                              --                                                                              --                                                                              5.5                                                                             4.7                                                                              9.5                                                                              15.0 1.0                                                                             .06                                                                              .17                                                                             .015                                                                             bal.                                      Mar-M509                                                                            -- 7.0                                                                             3.5                                                                             --                                                                              0.25                                                                             23.4                                                                             Bal.                                                                             --                                                                              --                                                                              .5 .6                                                                              -- 10.0                                      __________________________________________________________________________

A bond coat 12 lies over the substrate 10. The bond coat 12 is usuallycomprised of a MCrAlY alloy. Such alloys have a broad composition of 10to 35% chromium, 5 to 15% aluminum, 0.01 to 1% yttrium, or hafnium, orlanthanum, with M being the balance. M is selected from a groupconsisting of iron, cobalt, nickel, and mixtures thereof. Minor amountsof other elements such as Ta or Si nay also be present. These alloys areknown in the prior art and are described in U.S. Pat. Nos. 4,880,614;4,405,659; 4,401,696; and 4,321,311 which are incorporated herein byreference. The MCrAlY bond coat is preferably applied by electron beamvapor deposition though sputtering and low pressure plasma spraying mayalso be used.

Alternatively, the bond coat 12 can be comprised of an intermetallicaluminide such as nickel aluminide or platinum aluminide. The aluminidebond coat can be applied by standard commercially available aluminideprocesses whereby aluminum is reacted at the substrate surface to forman Aluminum intermetallic compound which provides a reservoir for thegrowth of an alumina scale oxidation resistant layer. Thus the aluminidecoating is predominately composed of aluminum intermetallic [e.g. NiAl,CoAl, FeAl and (Ni, Co, Fe)Al phases] formed by reacting aluminum vaporspecies, aluminum rich alloy powder or surface layer with the substrateelements in the outer layer of the superalloy component. This layer istypically well bonded to the substrate. Aluminiding may be accomplishedby one of several conventional prior art techniques, such as, the packcementation process, spraying, chemical vapor deposition,electrophoresis, sputtering, and slurry sintering with an aluminum richvapor and appropriate diffusion heat treatments. Other beneficialelements can also be incorporated into diffusion aluminide coatings by avariety of processes. Beneficial elements include Pt, Pd, Si, Hf andoxide particles, such as alumina, yttria, hafnia, for enhancement ofalumina scale adhesion, Cr and Mn for hot corrosion resistance, Rh, Taand Cb for diffusional stability and/or oxidation resistance and Ni, Cofor increasing ductility or incipient melting limits.

In the specific case of platinum modified diffusion aluminide coatinglayers, the coating phases adjacent to the alumina scale will beplatinum aluminide and/or nickel-platinum aluminide phases (on a Ni-basesuperalloy). Intermetallic bond coats are known in the prior art and aredescribed in U.S. Pat. No. 5,238,752 and copending U.S. patentapplication Ser. No. 06/603,811, now U.S. Pat. No. 5,514,482 which areincorporated herein by reference.

Through oxidation an alumina or aluminum oxide layer 14 is formed overthe bond coat 12. The alumina layer 14 provides both oxidationresistance and a bonding surface for the ceramic layer 16. The aluminalayer 14 may be formed before the ceramic layer 16 is applied, duringapplication of layer 16, or subsequently by heating the coated articlein an oxygen containing atmosphere at a temperature consistent with thetemperature capability of the superalloy, or by exposure to the turbineenvironment. The sub-micron thick alumina scale will thicken on thealuminide surface by heating the material to normal turbine exposureconditions. The thickness of the alumina scale is preferably sub-micron(up to about one micron).

The ceramic layer 16 is applied by electron beam vapor deposition and asresult has a columnar grained microstructure. The columnar grains orcolumns 18 are oriented substantially perpendicular to the surface ofthe substrate 10. Between the individual columns 18 are micron sizedgaps 20 extending from the outer surface 22 of the ceramic layer 16toward (within a few microns) of the alumina layer 14. The presence ofintercolumnar gaps reduces the effective modulus (increases compliance)of the stabilized zirconia layer in the plane of the coating. Increasedcompliance provided by the gaps enhances coating durability byeliminating or minimizing stresses associated with thermal gradient andsuperalloy/zirconia thermal expansion mismatch strains in the stabilizedzirconia layer. Alternatively, the ceramic layer 16 can be applied by aplasma spray process. Although this process does not produce a columnarmicrostructure, it does create an interconnected network of subcriticalmicrocracks with micron-width opening displacements, which reduce themodulus of the stabilized zirconia layer. The network of subcriticalmicrocracks performs the same function as the gaps 20. In thisapplication the term "gap" includes these microcracks.

The ceramic layer 16 may be any of the conventional ceramic compositionsused for this purpose. A preferred composition is the yttria stabilizedzirconia coating. These zirconia ceramic layers have a thermalconductivity that is about 1 and one-half orders of magnitude lower thanthat of the typical superalloy substrate such as MAR-M247. Instead of orin addition to the yttria, the zirconia may be stabilized with CaO, MgO,CeO₂ as well as Y₂ O₃. Another ceramic believed to be useful as thecolumnar type coating material within the scope of the present inventionis hafnia which can be yttria-stabilized. The particular ceramicmaterial selected should be stable in the high temperature environmentof a gas turbine. The thickness of the ceramic layer may vary from 1 to1000 microns but is typically in the 50 to 300 microns range.

Because of differences in the coefficients of thermal expansion betweenthe substrate 10 and the ceramic layer 16, when heated or cooled, thesubstrate 10 expands (or contracts) at a greater rate than the ceramiclayer 16. The gaps 20 allow the columnar grains 18 to expand andcontract without producing stresses that would cause the ceramic layerto spall or crack.

When exposed to temperatures over 1100° C. (2012° F.) for periods oftime, sintering of the columnar grains 18 occurs. The gaps 20 close asadjacent columnar grains 18 bond together. With the gaps 20 closed, theceramic layer 16 is less able to accommodate the thermal expansionmismatch and may spall or crack. Resistance to sintering is imparted tothe columnar grains 18 by sheathing them with a submicron layer of bondinhibitor 24. The bond inhibitor 24 is preferably unstabilized zirconiawhich will cycle through disruptive tetragonal and monoclinic phasetransformations every thermal cycle and thereby inhibit bonding ofadjacent grains 18. Unstabilized hafnia is another material that may beused as the bond inhibitor 24. It may also significantly increase thetemperature required for sintering because its melting temperature isabout 200° C. (392° F.) higher than that of zirconia. Pure hafnia alsohas a monoclinic structure which should bond poorly with the cubic phaseof the yttria stabilized zirconia grains 18.

The bond inhibitor 24 is applied by immersing the coated substrate intoa zirconia or hafnia sol gel bath. Most of the volume of the sol gel issolvent and evaporates when the substrate is removed from the bath.Consequently, the gaps 20 remain partially open which is necessary forstrain accommodation. Partial chemical vapor infiltration is anothermethod that can be used to apply the bond inhibitor 24.

Various modifications and alterations to the above described preferredembodiment will be apparent to those skilled in the art. Accordingly,this description of the invention should be considered exemplary and notas limiting the scope and spirit of the invention as set forth in thefollowing claims.

What is claimed is:
 1. A superalloy article having a ceramic thermalbarrier coating on at least a portion of its surface, comprising:asuperalloy substrate; a bond coat overlying the substrate and selectedfrom the group consisting of aluminides and MCrAlY where M is a metalselected from the group consisting of iron, cobalt, nickel, and mixturethereof; a stabilized ceramic coat overlying the bond coat and having anouter surface, the stabilized ceramic coat having a microstructurecharacterized by a plurality of micron sized gaps extending from theouter surface towards the bond coat; and an unstabilized ceramic bondinhibitor disposed in substantially all of the gaps for inhibitingbonding between the stabilized ceramic across said gaps.
 2. The articleof claim 1 further comprising a layer of alumina between the bond coatand the ceramic coat.
 3. The article of claim 1 wherein said bondinhibitor is selected from a group consisting of unstabilized zirconia,unstabilized hafnia, and mixtures thereof.
 4. The article of claim 1wherein said microstructure is a columnar grain microstructure.
 5. Thearticle of claim 1 wherein said aluminide is selected from the groupconsisting of nickel aluminide and platinum aluminide.
 6. A thermalbarrier coating system for a superalloy substrate, comprising:a bondcoat overlying the substrate and selected from the group consisting ofaluminides and MCrAlY where M is metal selected from the groupconsisting of iron, cobalt, nickel and mixtures thereof; a stabilizedceramic coat overlying the bond coat and having an outer surface, thestabilized ceramic coat having a microstructure characterized by aplurality of micron sized gaps extending from the outer surface towardsthe bond coat; and an unstabilized ceramic bond inhibitor disposed insubstantially all of the gaps for inhibiting bonding between thestabilized ceramic across said gaps.
 7. The article of claim 6 whereinsaid bond inhibitor is selected from a group consisting of unstabilizedzirconia, unstabilized hafnia, and mixtures thereof.
 8. The article ofclaim 1 wherein said stabilized ceramic coat is selected from a groupconsisting of stabilized zirconia, stabilized hafnia, and mixturesthereof.
 9. The article of claim 6 wherein said stabilized ceramic coatis selected from a group consisting of stabilized zirconia, stabilizedhafnia, and mixtures thereof.